The invention relates to a process for the preparation of metal surfaces for the subsequent application of organic coatings by applying a phosphate coating by means of wetting with a phosphatizing liquid, containing at least 2-valent cations, and subsequent drying in situ of the liquid film as well as to its use for the treatment of aluminum surfaces.
The so-called three-step processes are increasingly gaining in importance for the chemical surface treatment of metals, for example as preparation for the application of paints, adhesives and plastics. In the first step, the metal surface is cleaned in order to free it from oil, dirt and corrosion products. The second step represents rinsing with water, in which residues of chemicals from the first step are removed from the metal surface. Finally, in the third step, the metal surface is wetted with an aqueous chemical reaction solution and the liquid film dried in situ.
By means of the process described above, a thin non-metallic coat is formed on the metal, which can decisively improve surface quality, with suitably selected composition of the treatment liquid and reaction condition. Thus, for example, coatings of paints, adhesives and plastics can be distinguished by materially greater adhesion and considerably increased anti-corrosive protection, if they are applied to metal pretreated in this way.
West German Examined Patent Application (Auslegeschrift) No. 17 69 582, for example, describes a process, in which an aqueous solution, containing hexavalent chromium trivalent chromium alkali ions and silicon dioxide in definite quantitative ratios, is dried on the metal in situ. The coatings formed are very suitable, for example, as electrical insulation, as anti-corrosive protection and as primer for paints and the like.
Another process is known from U.S. Pat. No. 2,030,601, in which highly concentrated aqueous solutions, containing from 10 to 20% by weight of phosphoric acid and from 10 to 15% by weight of sodium dichromate, optionally with addition of silica, are brushed on to iron surfaces and subsequently dried in situ. This treatment serves for protection against rust formation.
Furthermore, it is known to produce coatings on metal surfaces with the aid of coating agents, containing a compound of hexavalent chromium and a polymeric organic substance,--so-called primers,--and subsequently dried in situ or stoved, (Euratom Patent Specification No. 197 164).
All the above-mentioned processes have the disadvantage in common that, as a result of the presence of 6-valent chromium, special precautionary measures are required in the application of the coating agent and in the handling of the coated metal and that, when metals, coated in this manner, are used as container material for foodstuffs and beverages, influence on the content of the container cannot be excluded. If the coating agents possess organic components, a further disadvantage is the low shelf-life (pot-life) of the treatment liquids.
In order to avoid the disadvantages connected with the use of treatment liquids containing 6-valent chromium, it is already known to wet the cleaned metal surface, especially of iron, zinc and aluminum, with an acidic aqueous solution containing chromium (III) ions, phosphate ions and finely divided silica, optionally also acetate ions, maleinate ions, zinc ions and/or manganese ions, and to dry the film of solution in situ, (West German Unexamined Patent Application (Offenlegungsschrift) No. 27 11 431). Although this process possesses considerable advantages over those mentioned above, it is a shortcoming that, when the coated metals are employed as a container material, a certain influence on foodstuffs and beverages cannot be totally excluded, as a result of the chromium (III)-content of the layer, and that the treatment liquid tends towards instability through the formation of sparingly soluble chromium phosphate.
Other patents of relevance are U.S. Pat. No. 3,450,577; U.S. Pat. No. 3,819,385; U.S. Pat. No. 2,502,441; and U.S. Pat. No. 3,586,543.
It is the object of the invention to provide a process, which avoids the known, especially the above-mentioned, shortcomings and can, nevertheless, be carried out in a simple manner and without additional expense.